The present invention relates in general to reducing NO.sub.x levels in power plant emissions and, in particular, to a new and useful system and method for providing combined thermal and fuel NO.sub.x control in a power plant utilizing furnace cleanliness and stoichiometric burner combustion.
In the power plant field, NO.sub.x formation from combustion processes using air has two components, thermal NO.sub.x and fuel NO.sub.x. The relative contribution of each depends primarily on the nitrogen content of the fuel and the temperature of the combustion process. NO.sub.x is produced at high temperatures by oxidation of the nitrogen from the combustion air (thermal NO.sub.x) and oxidation of the nitrogen from the fuel (fuel NO.sub.x). Thermal NO.sub.x is formed by gas-phase chain reactions between O.sub.2 radicals and N.sub.2.
NO.sub.x collectively refers to nitric oxide (NO), nitrogen dioxide (NO.sub.2) and nitrous oxide (N.sub.2 O). NO is the only nitrogen oxygen compound that can form, be stable, and exist in significant quantities in the high temperature portions of a utility boiler system.
Thermal NO.sub.x is NO derived from the heating of air. The three equations of the Zel'dovich mechanism illustrate the derivation of thermal NO.sub.x.
The first equation: EQU O.sub.2 +M&lt;-&gt;O+O+M (1)
shows the diatomic oxygen in the presence of a reaction medium M breaks down to or is combined from oxygen radicals. The forward reaction rate is proportional to temperature and on the order of 10 ft.sup.3 /lb-mol-hr at 2300.degree. F., 100 ft.sup.3 /lb-mol-hr at 2500.degree. F., and 1000 ft.sup.3 /lb-mol-hr at 2700.degree. F. The reverse reaction rate is constant on the order of 10.sup.14 ft.sup.3 /lb-mol-hr regardless of temperature.
If we use these forward and reverse reaction rate constants in the following equation for a reversible reaction:
A+B&lt;-&gt;C+D with forward rate k1 and reverse rate k2 at equilibrium: k1[A][B]=k2[C][D] PA1 fuel+heat-&gt;HCN, CN, OCN, HNCO, NH.sub.3, NH.sub.2, NH, or N.
Then, for this reaction proportionally few oxygen radicals exist relative to concentration of diatomic oxygen even at high temperatures. Also, since k1 increases with temperature and k2 does not, as temperature increases for a constant concentration of diatomic oxygen the radical concentration will increase.
The next equation: EQU N.sub.2 +O&lt;-&gt;NO+N (2)
shows that diatomic nitrogen in the presence of oxygen radicals will combine with some of those radicals to form NO and radical N.
The forward reaction rate constant for this equation is also proportional to temperature but on the order of 10.sup.6 ft.sup.3 /lb-mol-hr at 2600.degree. F. and 10.sup.7 ft.sup.3 /lb-mor-hr at 2900.degree. F. The reverse reaction rate is constant with temperature and on the order of 10.sup.15 ft.sup.3 /lb-mol-hr.
Although the reverse reaction rate constant is much higher than the forward rate constant, the high concentration of diatomic nitrogen relative to NO and N will force the reaction forward as long as sufficient oxygen radicals are present. At temperatures of 2700-2800 the first equation produces a sufficient oxygen radical concentration to drive this second reaction forward.
From these constants, the NO formation rate is also faster than the oxygen radical formation rate of the first equation. This means that the formation rate of NO is limited by the rate of formation of oxygen radicals which is proportional to temperature.
The third equation: EQU O.sub.2 +N&lt;-&gt;NO+O (3)
shows that if nitrogen radicals are present with diatomic oxygen, NO can be formed. The forward reaction rate is slightly temperature dependent and is on the order of 10.sup.14 ft.sup.3 /lb-mol-hr at 2600.degree. F. The reverse reaction rate is more temperature dependent but on the order of only 10.sup.9 ft.sup.3 /lb-mol-hr at 2600.degree. F. and 10.sup.10 ft.sup.3 /lb-mol-hr at 3000.degree. F. Both the forward reaction rate constant being higher than the reverse rate constant and the relatively diatomic oxygen concentration drive this reaction forward where N radicals exist.
Finally, by comparing the second and third equations, the reverse reaction rate of the second equation is on the order of 10.sup.15 ft.sup.3 /lb-mol-hr compared to the forward reaction rate of the last equation of 10.sup.14 ft.sup.3 /lb-mol-hr. This alone suggests that a nitrogen radical would preferentially combine with NO to form diatomic nitrogen and oxygen radicals over combining with diatomic oxygen to form NO. However, the difference in reaction rates constants is only on the order of 10 while the reaction of diatomic oxygen concentration to the concentration of NO will normally greatly exceed this factor. Therefore, NO is formed by both the second and third equations in the heating of air.
The other major mechanism for the formation of NO.sub.x is derived form the nitrogen in the fuel. Although oils and coals typically contain only 0.5 to 2% of nitrogen by weight, it is generally believed that fuel NO.sub.x contributes between 50 and 80% of the total NO.sub.x generated in unstaged firing applications.
The formation of fuel NO can be further divided into two paths dependent on the location of the nitrogen undergoing reaction: 1. volatile nitrogen which is released with the volatile matter of the coal, and 2. char nitrogen which remains with the char after devolatilization is complete. FIG. 1 shows the relative percentages of thermal, volatile based fuel, and char based fuel NO.sub.x generated versus heat rate and versus stoichiometry ratio for unstaged combustion (FIG. 1A).
The nitrogen in fuels is usually bound in the form of attached ammonia (NH.sub.3) or pyradine (C.sub.5 H.sub.5 N). Once the fuel is heated it breaks down into nitrogen bound intermediates:
These intermediates are released in the volatile mass leaving behind char. Then in the presence of oxygen these intermediates react forming NO as a product: EQU HCN+O.sub.x -&gt;NO+CO+. . . EQU NH.sub.3 +O.sub.x -&gt;NO+. . .
In the absence of sufficient oxygen these intermediates however react with any NO present to form diatomic nitrogen: EQU HCN+NO-&gt;N.sub.2 +. . . EQU NH.sub.3 +NO-&gt;N.sub.2 +H.sub.2 O
Experimental work has indicated that the conversion of HCN and NH.sub.3 was dependent on the local temperature and oxygen radical concentrations. The primary factor affecting fuel nitrogen conversion is the oxygen concentration. The formation rate constants are not as temperature dependent as the formation and reaction of oxygen radicals of the Zel'dovich mechanism for thermal NO.sub.x.
The char which remains after the volatile matter is released is high in carbon, low in oxygen and hydrogen, and contains some nitrogen, sulfur, and other minerals. Carbon and nitrogen oxidization of these char particles is heterogeneous as oxygen comes into contact with the hot particle surfaces: EQU O.sub.2 +N-&gt;NO+O
The conversion efficiency of char nitrogen is generally less than 20%, which is lower than for volatile nitrogen. This may relate to the known capability of carbon to reduce NO: EQU NO+C.sub.x -&gt;CO+N.sub.2
Federal regulations, namely, the Clean Air Act Amendments of 1990, Titles I and IV, mandate NO.sub.x reduction from stationary sources. The impact on utilities is that by the year 2000, more than 200,000 system MW must be retro-fitted with low-NO.sub.x systems. Title IV (acid rain) requires the use of low NO.sub.x combustion technology and Title I (ozone non attainment) requires RACT (reasonable, available control technology) to reduce NO.sub.x.
U.S. Pat. No. 4,408,568 to Wynnyckyj, et al. discloses a furnace wall ash monitoring system utilizing flux detectors. Heat fluxes are detected and converted to electrical signals indicating detected flux values which are displayed by traces on a charge plotted by an electronic recorder. These signals indicate the degree of furnace fouling and are used as a basis for sootblower actuation.
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